In recent years, packet transmission over Ethernet (registered trademark) is being used for a variety of purposes, and various kinds of data including audio, image, and moving image data are being transmitted thereby. The quality of service (QoS) to be satisfied by these various kinds of data differs according to the kind of data. Hence, packets that are constituted by the respective kinds of data have different priorities that depend on the kind of data.
Therefore, recently, some wireless packet transmission apparatuses that perform packet transmission have a packet priority control function that controls the order of transmitting packets with different priorities. FIG. 1 illustrates a common configuration of a packet priority controller that is applied to a wireless packet transmission apparatus that has a packet priority control function.
Packet priority controller 10 shown in FIG. 1 includes classification controller 11, schedule selector 12, and packet buffer 13.
Classification controller 11 classifies the priorities of packets to be transmitted that are input from a packet input section (unshown), in accordance with a priority that is set in a header of each packet. Classification controller section 11 stores packets that have undergone priority classification in transmission queues of packet buffer 13 that correspond to the respective priority classes.
Schedule selector 12 selects a priority control schedule in accordance with the packets that are stored in the transmission queues of packet buffer 13.
In this case, the term “priority control schedule” refers to a schedule that, with respect to packets with different priorities, sets a control scheme that determines the order of transmitting the packets for each packet.
Therefore, schedule selector 12 selects a priority control schedule by setting respective control schemes for packets with different priorities. For example, with respect to packets with the highest priority, schedule selector 12 sets a strict priority (SP) control scheme that transmits the packets with the highest priority irrespective of the existence of other packets, and with respect to other packets, schedule selector 12 sets a weighted round robin (WRR) control scheme that transmits the packets in an order that is determined by a ratio that is set by assigning weights that are in accordance with the priorities of the respective packets to a ratio at which the respective packets are transmitted.
In the following description, the term “priority control” refers to control of the order of transmitting packets with different priorities.
Packet buffer 13 performs readout of packets stored in transmission queues in accordance with the priority control schedule that is selected by schedule selector 12.
Thus, by including packet priority controller 10 that controls the order of transmitting packets with different priorities, the wireless packet transmission apparatus can transmit packets while taking into consideration the priorities of the packets.
In this case, packets for which the strict priority control scheme has been set are transmitted by the wireless packet transmission apparatus at a transmission rate to be satisfied by the packets. In contrast, packets for which the weighted round robin control scheme has been set are transmitted by the wireless packet transmission apparatus at a transmission rate that is determined after also taking into consideration a transmission rate to be satisfied by other packets with different priorities. In this case, although there may be instances where, depending on the transmission rate (hereunder, referred to as “wireless transmission rate”) at which the wireless packet transmission apparatus can perform packet transmission, the wireless packet transmission apparatus can not transmit packets at a transmission rate to be satisfied by packets for which the weighted round robin control scheme is set, the wireless packet transmission apparatus can transmit packets of all priorities. In this connection, the term “transmission rate” refers to a rate that is represented, for example, by bps (bits per second) that is a unit that shows a number of bits that are transmitted in one second or the like.
As described above, high priority packets for which the strict priority control scheme has been set are transmitted with priority irrespective of the existence of other packets.
Consequently, within the range of the wireless transmission rate of its own apparatus, the wireless packet transmission apparatus transmits high priority packets, for which the strict priority control scheme has been set, at a transmission rate (hereunder, referred to as “strict priority rate”) to be satisfied by packets for which the strict priority control scheme has been set.
Therefore, if the strict priority rate is higher than the wireless transmission rate, low priority packets are not transmitted at all, and communication of low priority packets is disconnected.
In this case, with a protocol such as TCP/IP in which an error is determined when it is detected that a state in which communication is disconnected has continued for a certain time period or more, because the determination of an error can be prevented if even a small number of packets are transmitted, complete disconnection of communication ought to be avoided even in the case of low priority packets.
In this connection, Patent Document 1 (JP2001-345815A) discloses a bandwidth guarantee control apparatus that guarantees a transmission rate in accordance with a priority of each packet even when the kinds or number of items of data that are transmitted vary.
More specifically, the bandwidth guarantee control apparatus disclosed in Patent Document 1 guarantees transmission rates to be satisfied by packets in the order of highest priority, and when transmission rates to be satisfied by low priority packets can not be guaranteed, among packets excluding high priority packets for which it is always necessary to satisfy a certain transmission rate, the bandwidth guarantee control apparatus increases the priority of packets for which the transmission rate could not be guaranteed. In accordance with the priority of the respective packets, the bandwidth guarantee control apparatus guarantees the transmission rate of each packet within a range of a remaining transmission rate that is obtained by subtracting the transmission rate to be satisfied by high priority packets from the wireless transmission rate.